TransferWise is currently only being used until it’s all in house. This is wrong, I stand corrected. It’s not been announced that they plan to bring it in house, sorry! The latest update from Monzo on this can be found below (thanks @Rat_au_van).
As for the savings pots, I’d rather they used a third party to pay that interest as I’m sure it’s more than they could pay. Makes sense to me.